Billel Kalache

Influence of Cu as a catalyst on the properties of silicon nanowires synthesized by the vapour–solid–solid mechanism

Unlike typical Au used as a catalyst for the synthesis of silicon nanowires via the vapour?liquid?solid mechanism, Cu has been found to induce a synthesis process governed by the vapour?solid?solid mechanism. Moreover, the temperature window for obtaining high-quality wires with Cu has been found to be relatively smaller than that shown by the Au: from 600 to 650??C. However, high-resolution transmission electron microscopy analysis reveals significant new properties of the nanowires obtained. They have the peculiarity of successively switching the silicon structure from diamond to the wurtzite phase along the growth direction. This change of the crystalline structure implies that it has an important impact on the transport properties and characteristics of electronic devices. The results will be important for the future integration and application of silicon, where electrical and thermal transport properties play a significant role.

Ion bombardment effects on microcrystalline silicon growth mechanisms and on the film properties

The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest th...

Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires

The occurrence of heterostructures of cubic silicon/hexagonal silicon as disks defined along the nanowire ⟨111⟩ growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. We observe that during growth, lamellar twinning on the cubic phase along the ⟨111⟩ direction is generated. Consecutive presence of twins along the ⟨111⟩ growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram.

Observation of Incubation Times in the Nucleation of Silicon Nanowires Obtained by the Vapor-Liquid-Solid Method

We report the observation of a characteristic incubation time in the growth of silicon nanowires using the vapor-liquid-solid growth mechanism. This incubation time manifests itself during the growth process as a characteristic time delay in the range of several seconds to minutes, prior to which no nanowires are formed. The observation is in excellent agreement with a theoretical model based on the diffusion of silicon through the catalyst, which predicts the presence of an incubation time, as determined by diffusion of the growth constituent through the solid catalyst. Furthermore the theoretical dependence of the incubation time on the activation energy is derived, and validated experimentally for the first time by measuring the incubation times of silicon nanowires obtained by chemical vapor deposition for both gold and copper as a catalyst. The experimentally observed incubation times are in excellent agreement with the theoretically predicted incubation times. The reported incubation times are a universal feature of vapor-liquid-solid growth and can be applied to any other metal/ semiconductor system for the synthesis of nanowires and provide a novel route to determine the phase space for nanowiresynthesis.

Numerical modeling of capacitively coupled hydrogen plasmas: Effects of frequency and pressure

In the field of plasma deposition of amorphous and microcrystalline silicon, the increase of the excitation frequency has often been considered as a way to enhance the deposition rate. Moreover, the increase of pressure has also been shown to enhance the deposition rate and improve the film properties. We attempt to clarify the effects of frequency in the 13.56–40.68 MHz range and to compare them to those of the pressure in the range of 0.5–1.5 Torr. For that purpose we use a numerical modeling of capacitively coupled hydrogen plasma, particularly relevant for the deposition of microcrystalline silicon. We use a one-dimensional time-dependent fluid model for the description of neutrals, positive and negative ions, and electrons, which involves a chemistry model taking into account 32 reactions in the gas phase and on the surface of the electrodes. The results of the model for the symmetrical system show that both pressure and frequency have pronounced influence on the parameters of the discharge: sheath thickness, ratio between power transferred to ions and electrons, and concentration and flux of atomic hydrogen at the electrode surface. We found that increasing the excitation frequency, while keeping constant the power dissipated in the plasma, leads to a more moderate increase of electron density as compared with the case of constant rf-voltage amplitude. The analysis of this phenomenon reveals that, with increase of frequency, the power coupling to the electrons becomes more efficient due to the decrease of the phase shift between voltage and current for both constant power and constant voltage conditions. There is, in addition, a significant drop of the sheath voltage with frequency when the power dissipated in the plasma is kept constant. This leads to the reduction in the drift loss rate for charged species. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species and, as a result, electron density enhancement is less pronounced. The increase of pressure leads to a more uniform spatial dissipation of the power coupled to the plasma, whereas the increase in frequency results in a higher amount of power dissipated on the plasma-sheath boundaries due to the decrease of the sheath width.

Investigation of coupling between chemistry and discharge dynamics in radio frequency hydrogen plasmas in the Torr regime

We present the results of a study of a capacitively coupled hydrogen discharge by means of a one-dimensional numerical fluid model and experiments. The model includes a detailed description of the gas-phase chemistry taking into account the production of H? ions by dissociative attachment of H2 vibrational levels. The population of these levels is described by a Boltzmann vibrational distribution function characterized by a vibrational temperature TV. The effect of the dissociative-attachment reaction on the discharge dynamics was investigated by varying the vibrational temperature, which was used as a model input parameter. Increasing the vibrational temperature from 1000 to 6000?K affects both the chemistry and the dynamics of the electrical discharge. Because of dissociative attachment, the H? ion density increases by seven orders of magnitude and the H? ion density to electron density ratio varies from 10?7 to 6, while the positive ion density increases slightly. As a consequence, the atomic hydrogen density increases by a factor of three, and the sheath voltage drops from 95 to 75?V. Therefore, clear evidence of a strong coupling between chemistry and electrical dynamics through the production of H? ions is demonstrated. Moreover, satisfactory agreement between computed and measured values of atomic hydrogen and H? ion densities gives further support to the requirement of a detailed description of the hydrogen vibrational kinetics for capacitively coupled radio frequency discharge models in the Torr regime.

Microcrystalline silicon: An emerging material for stable thin‐film transistors

Top-gate and bottom-gate microcrystalline-silicon thin-film transistors (TFTs) have been produced at low temperature (150-250°C) by the standard radio-frequency glow-discharge technique using three preparation methods: the hydrogen dilution of silane in hydrogen, the layer-by-layer technique, and the use of SiF 4 -Ar-H 2 feedstock. In all cases, a stable top-gate TFT with mobility values around 1 cm 2 /V-sec have been achieved, making them suitable for basic circuit on glass applications. Moreover, the use of SiF 4 gas combined with specific plasma treatments of the a-SiN:H dielectric produces large columns, even at the interface with the dielectric. This leads to stable bottom-gate TFTs, fully compatible with todays a-Si:H production facilities, reaching mobility values up to 3 cm 2 /V-sec. These devices are an interesting alternative to laser-crystallized polysilicon thin films in a growing number of applications.

Microcrystalline Silicon Thin-Films Grown by Plasma Enhanced Chemical Vapour Deposition - Growth Mechanisms and Grain Size Control

Besides plasma reactions, Plasma Enhanced Chemical Vapor Depositi on f silicon thin films at low temperatures involves surface and subsurface reacti ons of the impinging radicals and ions. Dilution of the silane feedstock in hydrogen or high dissociation of pur e silane results in a large flux of atomic hydrogen towards the substrate, which can induce a trans ition from amorphous to microcrystalline silicon growth. In this paper we review previous r esults based on the layer-by-layer technique, which demonstrate that the ratio of atomic hydrogen with respect to silicon radicals is the main parameter governing the nature of the films and allows for the growth of fully crystallized thin layers on various substrates. These studies highlight the importance of subsurface reactions on microcrystalline silicon formation. We show that the driving force for the formation of stable nuclei is the achievement of a highly porous and hydrogen-rich layer. The plasma i s also a source of ions which have varying effects on the film properties, depending on the energy and identity (H and SiHx ) of the impinging ions. We discuss the role of ion energy on the different sta ges of the growth and show that this can be used to control the grain size from a few nanometers up to few tens of nanometers. In particular, we highlight the role of ion energy on the different sta ges of the growth. Finally, we will present results concerning the use of silicon tetrafluoride as a f eedstock, which allow us to achieve polycrystalline silicon thin films ( ~ 100 nm thick) even at a substrate temperatur e of 200 °C.

Ion bombardment effects on the microcrystalline silicon growth mechanisms and structure

The role of ion bombardment in the growth mechanisms and the microstructure of microcrystalline silicon thin films produced by RF-PECVD is highlighted through the analysis of the effect of pressure and bias voltage applied to the RF electrode. The films are deposited on glass substrates by the standard hydrogen dilution of silane. It turns out that ion bombardment presents detrimental as well as beneficial aspects. In particular we show that high energy ions promote the nucleation of crystallites on glass substrates. Moreover, we show that after the nucleation phase the ion energy should be reduced to avoid damage to the bulk and the formation of an amorphised subsurface layer.

34.4: Invited Paper: Microcrystalline Silicon: An emerging Material for Stable Thin Film Transistors

Top gate and bottom gate microcrystalline silicon thin film transistors (TFTs) have been produced by the radio frequency glow discharge technique using three preparation methods: the standard hydrogen dilution of silane in hydrogen, the use of the layer-by-layer technique, and the use of SiF4-Ar-H2 feedstock. In all cases, stable top gate TFT with mobility values around 1 cm2/V.s have been achieved, making them suitable for basic circuit on glass applications. Moreover, the use of SiF4 gas combined with specific treatments of the a-SiN:H dielectric in bottom gate TFTs, fully compatible with todays a-Si:H production facilities, lead to an enhancement of the mobility which reaches stable values around 3 cm2/V.s.